1. Field of the Invention
The present invention relates to a press die which facilitates the smooth feeding of a blank (a piece of metal sheet cut to a size adequate for forming) at the time of press-forming automotive outer and inner panels.
2. Description of the Related Arts
For improved safety and environmental protection, automobile manufacturers have been attempting to make automotive bodies stronger and lighter by replacing conventional steel sheets with high-strength steel sheets or light-metal sheets (such as aluminum alloy sheets).
Unfortunately, high-strength steel sheets and light metal sheets usually have the disadvantage of being poor in press-formability. They easily become thin and crack during press-forming, and this prevents their general acceptance.
There have been proposed several techniques for preventing cracking in press-forming. It has been common practice to apply highly-lubricating rust preventing oil to blanks, to form solid lubricating film on blanks, and to use so-called press oil. These means are intended to prevent thickness reduction, to promote the material flow into a concave part, and to help the blank to readily slide on the surface of the blank holder.
On the other hand, there have been proposed several techniques for controlling the surface roughness of the press die, instead of applying press oil to blanks, in order to improve the slidability of blanks. For example, recent techniques disclosed in Japanese Patent Nos. 2711156, 2815417, and 2857497 are designed to roughen the blank holder surface by intentionally leaving machining marks, thereby reducing the sliding resistance of blanks. However, nothing is mentioned about the desirable direction of machining marks.
It is an object of the present invention to provide a new press die which permits the blank to readily slide, and hence prevents the blank from becoming thin, and offers good press-formability.
The present invention is directed to a press die comprising: a first die having a concave part, said concave part having an opening, the peripheral surface of the opening functioning as a first blank pressing face; a second die having a convex part; said second die forming a blank into a desired form in cooperation with said concave part; and a blank holder having a second blank pressing face, said blank holder working in such a way that said first blank pressing face and said second blank pressing face hold the blank between them while permitting it to flow into said concave part during pressing, wherein at least either of said first blank pressing face and said second blank pressing face has a roughened surface with a plurality of striated projections which are formed in the direction inclined in a range of xe2x88x9225xc2x0 to +25xc2x0 toward the circumferential direction of the edge of the opening of said concave part.
The present invention produces the following effects. The actual contact area between the blank and the first or second blank pressing face is reduced because the blank pressing face is roughened with a plurality of striated projections formed thereon. The reduced contact area causes the blank to experience less sliding resistance. The striated projections are formed in the direction inclined within xc2x125xc2x0 toward the circumferential direction of the edge of the opening of the concave part. Therefore, the blank comes into discontinuous contact with the blank pressing face. This prevents the blank from being stretched, deformed, and cracked during press forming. The resulting formed product has an accurate thickness.
The roughened face with striated projections should preferably have a surface roughness (Ra) of 0.02-60 xcexcm measured in the direction perpendicular to the circumferential direction of the edge of the opening of the concave part. Such a rough surface is effective in reducing sliding resistance further.